The present invention relates to a turbine, such as a gas turbine, turbo-supercharger, or the like. More particularly, the invention pertains to a turbine shaft for use in such an apparatus, and even more specifically, to a connecting portion between a ceramic shaft and a metal shaft of the turbine.
A conventional turbine of the same general type to which the invention pertains is illustrated in FIG. 1 of the drawings. The apparatus includes a turbine casing 5 and a compressor casing 7. A turbine shaft 1 extends longitudinally through the turbine casing 5 into the compressor casing 7. At the end of the turbine shaft 1 inside of the compressor casing 7 a compressor wheel 6 is mounted. An air inlet A is formed in the compressor casing 7 to supply air to the blades of the compressor wheel 6. The turbine shaft 1 is supported by a thrust bearing 10 and a bearing 3. Lubricating oil is supplied through an inlet B and collected in an oil resevoir 13. A lubricating oil outlet C is formed at the bottom of the reservoir 13.
A rotor 2 is mounted on a connecting portion 9 of the turbine shaft 1. A sealing ring 8', fitted in a groove 8, prevents oil from escaping around the turbine wheel 4 and exiting through the gas outlet E in the turbine casing 5. As is conventional, a gas inlet D communicates with the cavity around the rotor 2.
In this arrangement, the neck portion of the rotor 2 receives a large stress due to the relatively large diameter of the connecting portion 9. The larger the diameter of the connecting portion 9, the larger will be the required strength at its juncture with the rotor 2. Furthermore, the large diameter of the connecting portion 9 is disadvantageous in that the bearing speed is proportionally increased.
Recently, due to the very high temperatures present at the outlet, it has been proposed to fabricate the rotor 2 from a ceramic material. In this case, as illustrated in FIG. 2 of the drawings, a straight portion 2a of a ceramic rotor 2 is fitted into a cupped end 1a of a steel turbine shaft 1. Besides improving the resistance to heat, the weight of the turbine wheel 4 and rotor 2 is decreased, and the responsiveness thereof is hence improved. In such an arrangement, however, manufacturing problems arise. Particularly, heat treatment of the cupped end 1a, which must be in contact with a bearing, is difficult. Moreover, if the juncture 12 of the straight portion 2a is exposed at the left side of the sealing ring 8', as illustrated in FIG. 3, due to the difference in coefficients of expansion between ceramic and metal materials, it is difficult to maintain the brazed connection between the ceramic and metal members.
Accordingly, it is an object of the present invention to eliminate the above-discussed disadvantages inherent in prior art turbine shafts.